PIR D203S circuit op amp circuit doubt

[lok]

Hi all,

I am planning to do a PIR motion sensor with D203S. I have gone through the application circuit they provided..I didn't get why they connected R4 and C4 to the inverting input of the first opamp. In my observation they have done transimpedance amplifier...can anybody tell the significance of R4 and C4.

 

[d123]

Hi,

If it's not part of a filter, it may be for AC coupling, looking at the output capacitor.

 

[FvM]

I don't identify a transimpedance amplifier. There are two voltage amplifier stages, first non-inverting, second inverting. Both are incorporating high- and low-pass filter functions, or in terms of AC circuit analysis poles and zeros.

R4/C4 represents a zero, R3/R4 + 1 sets the mid frequency (pass band) gain of the first stage. C4 is also required by nature of the single supply amplifier topology.